1) i have a question about FET datasheets. are the graphs of Ids as f(Vds) with no load attached? as example, the IRFZ24N graph shows this function as 20us pulse. more Vds yieds more Ids (makes sense with no load attached). what does this graph mean?

my understanding of nFET is that Ids is a function of Vgs. does Ids rely on Vds if there is sufficient Vgs to cause full saturation? in other words, in my attached schematic if Vgs=10v and Vds=0.2v does the nFET conduct the load fully?

2) i need to switch a load between different current sense resistors (r1, r2, r3) or parallel them for different values. the fet gates would be switched using a dip switch. my current monitoring circuit (integrated into the LM1949) needs to read the voltage across the sense resistor. so if i monitor between fet common drain and ground the resistence will be fet(Rds-on) + sense, is this correct? the range of sense resistors (without fet) would be 0.1, 0.9, 0.8, 0.7, 0.6 (or parallel combinations). so there is a obvious issue of monitoring at the fet drain. could i use a multi nFET IC for each switch and parallel the FET's to reduce Rds?? any thoughts on a better way to switch the sense resistors in?

with a single sense resistor the LM1949 will do peak&hold or just hold as follows (peak is typically about 1ms):
R(ohms) peak/hold amps
.10 is 3.85/0.94
.09 is 4.27/1.11
.08 is 4.81/1.25
.07 is 5.50/1.43
.06 is 6.42/1.67

one last question. if Vds=0.021v and Vgs=10v does the mosfet conduct fully? the graph starts at Vds=0.1v. is it the case that if mosfet gate is driven hard Ids will ramp up irregardless of Vds? or at very low Vds, the current profile becomes f(Vds)=Ids?

I was thinking that you could use MOSFETs to switch the sense resistors (as you are proposing), and add a multiplexer to select which resistor is read by the LM1949. More parts, but the Rds(on) is taken out of the measurement.

I was thinking that you could use MOSFETs to switch the sense resistors (as you are proposing), and add a multiplexer to select which resistor is read by the LM1949. More parts, but the Rds(on) is taken out of the measurement.

Click to expand...

yes, this was another thought i had, but using a shunt jumper instead.